Traditional methods for building divided wall columns for use in heat and/or mass transfer processes required welding a dividing wall in an appropriate location directly to a pressure shell column and then proceeding with the installation of packing or distillation trays on either side of the divided wall column. Other methods for building divided wall columns required that half-cylindrical shells be welded onto the dividing wall itself. Both methods required a welder, working inside the pressure shell column, to place welds on the interior wall of the column and the dividing wall to prevent leaks between these members.
The previous methods of manufacture and designs of divided wall columns for smaller sized applications, however, are problematic for several reasons.
First, use of a divided wall column incorporating a traditional dividing wall subjects the dividing wall to both pressure and temperature differences on both sides of the divided wall column. These temperature and pressure differences arise due to the very nature of divided wall column operation. On either side of the dividing wall, different mass transfer conditions are created by the varying liquid and vapor flows through the column sections. These diverse mass transfer conditions yield different composition profiles along the length of the divided wall sections. The different composition vapors and liquids having distinct saturation temperatures thus create temperature differences on either side of the dividing wall.
The temperature differences create significant mechanical stresses on both the dividing wall as well as the pressure shell column, and as such, have to be carefully monitored and managed during plant operation. The pressure and temperature differences especially are significant during operating upsets, for example, during the startup or shut down of the facility.
These upset conditions can be especially problematic when the operating temperatures of the column system deviate substantially from ambient conditions. For example, an air separation column operates under cryogenic conditions with temperatures reaching −195° C. At that temperature, care must be taken while cooling down the columns to ensure that both sides of the columns are cooled at a uniform rate (i.e., to prevent one column from cooling at a rate much faster than the opposite side of the column).
To combat these mechanical stresses, support members or stiffening members have been incorporated to prevent buckling, for example. As illustrated in U.S. Pat. No. 7,357,378, incorporated herein by reference in its entirety, these support members assist in withstanding the pressure differentials and minimizing the effect of the temperature differentials. Use of these support or stiffening members, however, may increase the cost and have a detrimental effect on the efficiency of the distillation process.
Increasing the thickness of the dividing wall to compensate for the mechanical stresses has also been attempted, however, the increase in strength realized is minimal, especially in columns having large diameters. Further, increasing the diameter of the dividing wall also leads to complications associated with the welding of the dividing wall to the column wall. Increasing the thickness of the dividing wall also causes occupation of a greater portion of the column area leading to less efficient use of the available area.
Use of double (laminated) or honeycombed walls, strengthening ribs, or using distillation trays as stiffeners to strengthen the dividing wall all suffer from the same drawbacks described above.
Second, with the increased demand for divided wall columns to be smaller, the space available for a welder to enter the pressure shell column and weld the dividing wall to the pressure shell column or the half-cylindrical shell to the dividing wall itself is problematic. For example, welding a dividing wall in columns with smaller diameters creates a measurably “tighter” clearance for the welder and welding tools to fit and work. In addition, welding shims, support members and/or stiffening members inside these small traditional divided wall columns is increasingly difficult.